1. Field of the Invention
The present invention relates to a radiographic image conversion panel, a method for manufacturing the radiographic image conversion panel, a method for forming phosphor particles, a method for forming a photostimulable phosphor precursor, a phosphor precursor and a photostimulable phosphor.
2. Description of Related Art
In earlier technology, so-called radiography in which a silver salt is used in order to obtain a radiographic image has been utilized. However, a method for imaging a radiological image without using a silver salt has been developed. That is, a method for imaging by absorbing a radiation ray transmitted through a subject in a phosphor, thereafter, exciting the phosphor with a certain type of energy, and radiating the radiographic energy accumulated in the phosphor as a fluorescence is disclosed.
Concretely, a radiographic image conversion method in which a panel provided with a photostimulable phosphor layer on a support and either or both of visible ray and infrared ray is used as excitation energy has been known (see U.S. Pat. No. 3,859,527 specification).
As radiographic image conversion methods using photostimulable phosphors having higher luminance and higher sensitivity, a radiographic image conversion method using a BaFX:Eu2+ system (X: Cl, Br, I) phosphor (for example, see Japanese Patent Laid-Open Publication No. Sho 59-75200), a radiographic image conversion method using an alkali halide phosphor (for example, see Japanese Patent Laid-Open Publication No. Sho 61-72087), and an alkali halide phosphor containing metals of Tl+, Ce3+, Sm3+, Eu3+, Y3+, Ag+, Mg2+, Pb2+, In3+ as co-activators (for example, see Japanese Patent Laid-Open Publications Nos. Sho 61-73786 and Sho 61-73787) are developed.
Furthermore, recently, in analysis of diagnostic imaging, a radiographic image conversion panel having higher sharpness has been required. As a method for improving the sharpness, for example, attempts for improving sensitivity and sharpness by controlling the shape of photostimulable phosphors (hereinafter, also referred to as phosphors) have been made.
As one of these attempts, for example, there is a method for using a photostimulable phosphor layer having a fine quasi-columnar block formed by depositing a photostimulable phosphor on a support having a fine concavoconvex pattern (for example, see Japanese Patent Laid-Open Publication No. Sho 61-142497).
Further, a method for using a radiographic image conversion panel having a photostimulable phosphor layer in which cracks between columnar blocks obtained by depositing a photostimulable phosphor on a support having a fine pattern are shock-treated to be further developed (for example, see Japanese Patent Laid-Open Publication No. Sho 61-142500), further, a method for using a quasi-columnar radiographic image conversion panel in which cracks are caused from the surface side of a photostimulable phosphor layer formed on a support (for example, see Japanese Patent Laid-Open Publication No. Sho 62-39737), furthermore, a method for providing cracks by forming a photostimulable phosphor layer having a void on a support according to deposition, and thereafter, by growing the void according to heat treatment (for example, see Japanese Patent Laid-Open Publication No. Sho 62-110200), and the like are suggested.
Furthermore, a radiographic image conversion panel having a photostimulable phosphor layer in which an elongated columnar crystal having a constant slope to a normal line direction of a support is formed on the support according to a vapor phase deposition method (for example, see Japanese Patent Laid-Open Publication No. Hei 2-58000) is suggested.
Any of these processes of controlling shapes of the photostimulable phosphor layer is characterized in that since the transversal diffusion of stimulating excitation light or stimulated fluorescence can be suppressed, by rendering the photostimulable phosphor layer columnar (the light reaches the support surface while repeating reflection in a crack (columnar crystal) interface), the sharpness of images formed by the stimulated fluorescence can be noticeably increased.
Recently, a radiographic image conversion panel using a photostimulable phosphor in which Eu is activated to a ground material of alkali halide such as CsBr or the like is suggested. Particularly, it became possible to derive a high X-ray conversion efficiency, which was unable to be obtained in earlier technology, by using Eu as an activator.
However, diffusion of Eu according to heat is remarkable, and there is a problem such that the dispersion of Eu is easily caused and the existence of Eu in a ground material is distributed unevenly since the vapor pressure under vacuum is also high. Thereby, it has not yet been in practical use at market since it is difficult to activate it by using Eu and to obtain a high X-ray conversion efficiency.
Particularly, in activation of rare-earth element which is excellent in a high X-ray conversion efficiency, with respect to deposited film formation under vacuum, uniformizing is more difficult problem than vapor pressure property. Further, in manufacturing method, there is a problem such that the existence state of the activator becomes nonuniform since a number of heat treatments, such as heating of raw materials when preparing the photostimulable phosphor layers, heating of substrates (supports) at the time of vacuum deposition, and anneling (strain relaxation of substrates (supports)) treatment after film formation, is performed to these photostimulable phosphor layers formed by vapor phase growth (deposition). Further, there is a problem relating to the durability thereof.
Therefore, there have been demanded improvements in luminance, sharpness and durability which are demanded from a market as the radiographic image conversion panel.
On the other hand, particularly, in activation by a rare earth element which ensures high X-ray conversion efficiency, when forming a vapor deposition film in a vacuum, the heating during the vapor deposition generates a radiation heat on a substrate to exert an effect on a heat distribution of the substrate.
This heat distribution varies also depending on a degree of vacuum, and the crystal growth becomes uneven by the heat distribution to cause a rapid disturbance in the luminance and the sharpness, so that it is difficult to control these performances in the vacuum deposition film formation method.
When using a phosphor crystal prepared by using an alkali halide as the ground material, the performance as a phosphor is brought out by a single crystal forming method according to a vapor phase deposition method (a vacuum deposition method) or a pull method, and the phosphor crystal is sealed in a glass or metal case due to low moisture resistance thereof.
In the CsBr:Eu phosphor radiographic image conversion panel manufactured by using a vacuum deposition method, there are problems that the Eu cannot be stably diffused in a vacuum conditions at the formation described above and that the phosphor has a large limitation on the handling because it is sealed in a glass case due to low moisture resistance thereof and therefore, has difficulties in use for general purposes.
However, Eu has properties that diffusion by heat is remarkable and also the vapor pressure in a vacuum is high, so that there arises a problem that Eu is unevenly distributed in a ground material because it is easily dispersed in the ground material. Accordingly, it is difficult to activate a phosphor using Eu to attain high X-ray conversion efficiency and therefore, the method is not put into practical use on a market.
In the rare earth element activator which ensures high X-ray conversion efficiency, when employing the vacuum deposition film forming method, the heating during the vapor deposition generates a radiation heat on a substrate to exert an effect on a heat distribution of the substrate.
This heat distribution varies also depending on a degree of vacuum, and the crystal growth becomes uneven by the heat distribution to cause a rapid disturbance in the luminance and the sharpness, so that it is difficult to control these performances in the vacuum deposition film forming method (e.g., see Japanese Patent Laid-Open Publication No. H10-140148 and Japanese Patent Laid-Open Publication No. H10-265774). Accordingly, the vacuum deposition film forming method has problems in that, particularly, in the case of using the rare earth elements such as Eu, Eu cannot be stably diffused and the phosphor has a large limitation on the handling because it is sealed in a glass case due to low moisture resistance thereof. Further, the method is lacking in versatility because the raw material utilization efficiency is as low as only several % to 10%, resulting in high cost due to the low utilization efficiency.
Accordingly, in the market, there have been demanded improvements in production uniformity agreeing with the improvements of stability, luminance and sharpness which are required as a radiographic image conversion panel.